This work has three main goals: (i) Frameshift mutations and gamma-ray-induced deletions that abolish expression of HLA-DR antigens will be intruduced into human lymphoblastoid cells (LCL) in order to describe relations between DR and D-region determinants that stimulate primed lymphocyte (PL) reactivity. Deletion of entire haplotypes by gamma-irradiation followed by immunoselection will create a collection of LCL clones, each of which expresses one of the many existing haplotypes. Single haplotype mutants from which the GLO locus has been deleted, too, will help define specificity of human primed lymphocyte clones, which are becoming important in histocompatibility testing and in studies of disease associations. Mutants of a heterozygous LCL that have a gamma-ray-induced deletion of one entire haplotype or the other will be fused in order to determine if the hybrids express PLT specificities that result from allelic and/or non-allelic intractions. Additional frameshift and deletion mutations will be introduced into single haplotype mutants to create homozygous deficiencies of function and, in some cases, DNA. (ii) DNA from homozygous, gamma-ray-induced mutants that do not express DR antigens will be used to enrich for DR-mRNA from normal cells by means of DNA-RNA reassociation and fractionation for unreassociated mRNA. The DR-enriched mRNA will be used to make cDNA probes or libraries of cDNA clones, which will be used to screen libraries of genomic DNA clones for clones that contain D-region genes. (ii) Probes made with DNA clones of the A or B locus will be used to isolate from heterozygous A-deletion and B-deletion mutants DNA segments that are scattered throughout the MHC. These DNA clones will be bases for walking along the DNA and, finally, determining the nucleotide sequence of the entire MHC and its relation to function.